Previous studies had indicated that microsomal NADPH oxidase activity produced radicals possessing properties of .OH. current studies indicate that in addition to this apparent radical, singlet oxygen (1O2) may also be generated by the enzymic activity. The data indicate that the oxidation of NADPH by microsomes produces superoxide anion (O2) which can then react with hydrogen peroxide produced by NADPH oxidation yielding .OH, or else the O2 may undergo chemical dismutation to yield 1O2. The formation of these highly reactive forms of oxygen (.OH and 1O2) appears to depend upon traces of inorganic iron being present during the enzymic activity.